The bullous pemphigoid antigen BPAG1 is required for keratin filament linkage to the hemidesmosome, an adhesion complex in epithelial basal cells. BPAG1 structural organization is similar to the intermediate filament-associated proteins desmoplakin I (DPI) and plectin. All three proteins have predicted dumbbell-like structure with central alpha-helical coiled-coil rod and regions of N- and C-terminal homology. To characterize the size of the N-terminal globular domain in BPAG1, two polypeptides spanning possible boundaries with the coiled-coil rod domain of BPAG1 were expressed in Escherichia coli. BP-1 (Mr = 111,000), containing amino acids 663-1581 of BPAG1 (Sawamura, D., Li, K., Chu, M.-L., and Uitto, J. (1991) J. Biol. Chem. 266, 17784-17790), and BP-1A, with a 186 amino acid N-terminal deletion, were purified. BP-1 and BP-1A behave as highly asymmetric dimers in aqueous solution according to velocity sedimentation and gel filtration. Both have globular heads with rod-like tails of roughly equal length, 55-60 nm, upon rotary shadowing. BP-1A content of alpha-helix, determined by circular dichroism, is approximately 90%, consistent with alpha-helical coiled-coil formation in the rod-like tails. The estimated rod length, 383 +/- 57 amino acids (0.15 nm/amino acid), implies that globular folding in the BPAG1 N-terminal extends to the end of N-terminal homology with DPI and plectin. These findings support the existence of a common domain structure in the N-terminal regions of the BPAG1/DPI/plectin family.